Move /src to /subprojects

This will facilitate breaking up Nix into multiple packages for each
component with Meson.

subprojects/libfetchers/attrs.hh

@@ -0,0 +1,44 @@
+#pragma once
+///@file
+
+#include "types.hh"
+#include "hash.hh"
+
+#include <variant>
+
+#include <nlohmann/json_fwd.hpp>
+
+#include <optional>
+
+namespace nix::fetchers {
+
+typedef std::variant<std::string, uint64_t, Explicit<bool>> Attr;
+
+/**
+ * An `Attrs` can be thought of a JSON object restricted or simplified
+ * to be "flat", not containing any subcontainers (arrays or objects)
+ * and also not containing any `null`s.
+ */
+typedef std::map<std::string, Attr> Attrs;
+
+Attrs jsonToAttrs(const nlohmann::json & json);
+
+nlohmann::json attrsToJSON(const Attrs & attrs);
+
+std::optional<std::string> maybeGetStrAttr(const Attrs & attrs, const std::string & name);
+
+std::string getStrAttr(const Attrs & attrs, const std::string & name);
+
+std::optional<uint64_t> maybeGetIntAttr(const Attrs & attrs, const std::string & name);
+
+uint64_t getIntAttr(const Attrs & attrs, const std::string & name);
+
+std::optional<bool> maybeGetBoolAttr(const Attrs & attrs, const std::string & name);
+
+bool getBoolAttr(const Attrs & attrs, const std::string & name);
+
+std::map<std::string, std::string> attrsToQuery(const Attrs & attrs);
+
+Hash getRevAttr(const Attrs & attrs, const std::string & name);
+
+}